1. Field of the Invention
The present invention relates to an exposure apparatus such as an X-ray stepper for, e.g., semiconductors requiring manufacturing rules on the order of micrometers or less, and the method of detecting a relative positional displacement between a mask and a wafer in the apparatus.
2. Related Background Art
The degrees of integration of semiconductor elements such as LSIs keep on increasing, and g- and i-line steppers are commercially available at present. Furthermore, exposure apparatuses, X-ray steppers, and the like using an excimer laser are beginning to make the transition from the study phase to the practical application phase. Such an exposure apparatus can achieve a line width resolution of 0.25 .mu.m to 0.1 .mu.m. However, the relative position alignment precision between a mask and a wafer required for achieving the above-mentioned resolution is at least 1/10 the resolution, i.e., 30 to 10 nm. As the high-precision position alignment method, a double diffraction grating method has been proposed [Flanders et. al, "Appl. Phys. Lett. 31, 426 (1977)"]. FIG. 1 shows the principle of this method. In this method, diffraction gratings land 2 are respectively provided to a mask 3 and a wafer 4, laser beams are projected on these gratings, and the interference light intensities of a plurality of diffracted light components from these gratings are detected, by detectors 6 and 7 which transmits a signal to a calculator 8 which generates an alignment signal thereby detecting the relative positional displacement between the mask 3 and the wafer 4, and aligning them according to their relative position.
However, in the above-mentioned conventional double diffraction grating method, the alignment precision is determined by the formation precision of the diffraction gratings on the mask and the wafer. In the current state of the art, the pitch of the diffraction gratings is accurate up to 1 .mu.m, and the formation precision of the gratings is accurate up to 0.1 .mu.m. Even if the averaging effect corresponding in number to the grating marks contributing to light diffraction is taken into consideration, the limit of the alignment precision is 10 nm. Therefore, in an exposure apparatus used in micro-processes of semiconductors whose degree of integration is expected to further increase in the future, a method of aligning the relative position between a mask and a wafer with a precision on the order of 1 nm is demanded.
Meanwhile, an interatomic force microscope (to be abbreviated to as an "AFM" hereinafter) has been [Binnig et. al, Phys. Rev. Lett. 56, 930 (1986)], so that three-dimensional pattern information (unevenness information) on a surface can be obtained on the atomic.sup.. molecular order. In the AFM, a force acting between a sample and a probe is detected from a flexure amount of a cantilever, caused by the force, which comprises an elastic member for supporting the probe and which is set near the sample surface within a distance of 1 nm or less thereto. More specifically, in the AFM, the sample surface is scanned while controlling the distance between the sample and probe so as to keep the detected force constant, whereby the three-dimensional pattern on the surface is observed with a resolution on the order of 1 nm or less. Since the sample need not have a conductivity, the AFM can observe an insulative sample, in particular, a semiconductor resist surface, a biological high polymer, and the like on the atomic.sup.. molecular order.